Let me elaborate a bit more on the Lars' answer... erm.. I mean comment.

Your range will be divided into n bands each of width (size of range / number of bands = n) where n is the number of datum within the given data set. So if you have 100px and 10 datum you'll have that 100px/10 datum = 10px wide bars. That's why "You can't have bands with uneven width" - Lars <3

You can read more about rangeBands and rangeRoundBands here:

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